The present invention relates to a plug-in type electronic control unit used for controlling various mechanical systems, as well as a structure of connection of a wiring board with a plug member, a unit of connection of the electronic part with the wiring board, and a process for mounting the electronic part, which are technically associated with the plug-in type electronic control unit.
There is a conventionally known plug-in type electronic control unit of the above-described type, which includes a wiring board accommodated in a case, a plurality of electronic parts mounted to the wiring board and also accommodated in the case, and plug members integrally provided on opposite sides of the case. In this case, connecting areas are provided in the wiring board on opposite sides of each of electronic part-mounted areas, and a plurality of connecting terminals existing in each of the connecting areas and a plurality of pin-shaped contacts of each plug member are electrically connected to each other by utilizing a wire bonding process.
If the plug-in type electronic control unit is constructed in the above manner, however, the following problem is encountered: The size of the wiring board is increased, and since the plug members are disposed on the opposite sides of the wiring board, the size of the entire electronic control unit is increased.
There is a conventionally known unit of connection of an electronic part with a wiring board, which includes an electronic part provided with at least one electrode having a projection (bump) formed by application of a wire bonding process, an electrically insulating wiring board having at least one connecting terminal opposed to the projection, a conductive bonding material such as a solder or the like which electrically bonds the projection and the connecting terminal to each other, and an electrically insulating sealing agent penetrated into a gap between the electronic part and the wiring board by a capillary phenomenon to bond the electronic part and the wiring board to each other.
The sealing agent is used to electrically insulate a connection comprised of the projection of the electrode, the connecting terminal and the conductive bonding material from the surroundings and to moderate the thermal stress acting on the connection due to a difference in linear expansion coefficient between the electronic part and the wiring board in the course of cooling after the electronic part has operated.
In the conventional connection unit, however, the gap between the electronic part and the wiring board is extremely narrow, and the volume of the sealing agent is very small. Therefore, when a thermal stress is applied to the connection due to the difference in linear expansion coefficient between the electronic part and the wiring board in the course of cooling after operation of the electronic part, the following problem is encountered: the thermal stress cannot be sufficiently moderated and as a result, there is a possibility that the connection may be broken.
Further, there is a conventionally known process for mounting an electronic part, which includes forming a projection (bump) on an electrode of an electronic part by utilizing a wire bonding technique in correspondence to the connection unit, electrically connecting the projection with a connecting terminal of a wiring board using a conductive bonding material such as a solder or the like, and then allowing a liquid-state electrically insulating sealing agent to be penetrated into a gap between the electronic part and the wiring board by a capillary phenomenon, thereby bonding the electronic part and the wiring board to each other.
With the conventional process, however, the connection of the projection of the electrode with the connecting terminal and the application of the sealing agent to the connection are carried out at separate steps, resulting in problems of a bad workability and an increased mounting cost.
Accordingly, it is an object of the present invention to provide a plug-in type electronic control unit of the above-described type, a reduction in size of which is realized by utilizing a surface of the wiring board opposite from the electronic part-mounted surface as a plug member-mounted surface and by employing a particular electrically connecting system.
To achieve the above object, according to the present invention, there is provided a plug-in type electronic control unit comprising a wiring board, a plurality of electronic parts mounted on one of opposite surfaces of the wiring board by utilizing a wireless bonding process, and a plug member mounted on the other surface of the wiring board by utilizing a wireless bonding process.
If the plurality of electronic parts, the wiring board and the plug member are constructed in a laminated structure as described above, the planar extent of the electronic control unit can be remarkably suppressed as compared with the conventional electronic control unit. In addition, if the wireless bonding process, namely, a connecting process using no wire, is employed to mount the electronic parts and the plug member, the planar extent of the electronic control unit in a laminating direction can be remarkably suppressed. In this manner, it is possible to realize a reduction in size of the plug-in type electronic control unit.
It is another object of the present invention to provide a structure of connection of a wiring board with a plug member, wherein the thermal stress acting on a connection between a connecting terminal of the wiring board and a pin-shaped contact of the plug member can be sufficiently moderated.
To achieve the above object, according to the present invention, there is provided a structure of connection of a wiring board with a plug member, comprising a wiring board having an electronic part mounted on one of opposite surfaces thereof and a connecting terminal on the other surface thereof, a plug member including a pin-shaped contact opposed to the connecting terminal, a conductive bonding material which electrically connects the connecting terminal and the pin-shaped contact to each other, and an electric insulator which surrounds the conductive bonding material and is sandwiched between the wiring board and the plug member.
In this connection structure, it is possible to increase the volume of the electric insulator existing between the wiring board and the plug member, thereby sufficiently moderating the thermal stress acting on the connection due to the difference in linear expansion coefficient between the wiring board and the plug member in the course of cooling after of the electronic part has operated.
It is a further object of the present invention to provide a unit of connection of the electronic part with the wiring board, wherein a thermal stress acting on a connection comprised of a projection of the electrode, a connection terminal and a conductive bonding material can be moderated sufficiently.
To achieve the above object, according to the present invention, there is provided a unit of connection of an electronic part with a wiring board, comprising an electronic part provided with at least one electrode having a projection, a wiring board including at least one connecting terminal opposed to the projection, a conductive bonding material which electrically connects the projection and the connecting terminal to each other, and an electrically insulating sealing agent which surrounds the conductive bonding material and bonds the electronic part and the wiring board to each other, the relationship between a diameter a of the projection and a thickness b of the sealing agent existing between the electronic part and the wiring board being set at b greater than a, and a distance between a tip end of the projection and the connecting terminal being set in a range of c greater than 0.02 mm.
In the connection unit of the above-described arrangement, the volume of the sealing agent existing between the electronic part and the wiring board is large, and the distance c is large. Therefore, a thermal stress acting on the connection due to a difference in linear expansion coefficient between the electronic part and the wiring board in the course of cooling after operation of the electronic part can be moderated sufficiently.
This is because, although a lateral shearing force is applied to the sealing agent existing between the electronic part and the wiring board substantially in parallel to the opposed surfaces of the electronic part and the wiring board due to the difference in linear expansion coefficient, the shearing force per unit volume acting on the sealing agent can be decreased when the volume of the sealing agent is large as described above, and in addition, the connection sufficiently withstands such shearing force because of the large distance c. Thus, the connection unit has an excellent thermal shock resistance.
It is a yet further object of the present invention to provide a process for mounting the electronic part, which is capable of producing a connection unit, wherein the operability of mounting of the electronic part can be improved to reduce the mounting cost, and a thermal stress acting on the connection can be moderated sufficiently.
To achieve the above object, according to the present invention, there is provided a process for mounting an electronic part having at least one electrode onto a heat resistant and electrically insulating wiring board having at least one connecting terminal, while connecting the electrode electrically to the connecting terminal, the process comprising the steps of bringing an electrically insulating skin film-shaped sealing agent into close contact with the wiring board, filling a conductive bonding material into at least one connecting bore in the skin film-shaped sealing agent which corresponds to the connecting terminal, and bonding the electronic part to the wiring board through the skin film-shaped sealing agent in a state in which a projection of the electrode of the electronic part has been put into the conductive bonding material in the connecting bore.
In the above process, the connecting operation of the projection of the electrode with the connecting terminal and the application of the sealing agent to a connection comprised of the projection, the connecting terminal and the conductive bonding material are performed at the same step. Therefore, the operability of mounting the electronic part can be improved to reduce the mounting cost.
In addition, since the skin film-shaped sealing agent is interposed between the electronic part and the wiring board, the volume of the sealing agent existing between the electronic part and the wiring board can be increased in the connection unit. Thus, it is possible to sufficiently moderate the thermal stress acting on the connection due to a difference in linear expansion coefficient between the electronic part and the wiring board in the course of cooling after the electronic part has operated.
This is because, although a lateral shearing force acts on the sealing agent between the electronic part and the wiring board substantially in parallel to the opposed surfaces of the electronic part and the wiring board due to the difference in linear expansion coefficient, in the case where the sealing agent has a large volume as described above, the shearing force per unit volume acting on the sealing agent can be reduced. Thus, the connection unit has an excellent thermal shock resistance.